Bridgewater
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bridgewater clear sky chart from 2005-11-06 to 2025-04-26.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	20% 11% 4 8% 5% 4 4 3 5% 6% 29%	114418
	Night	20% 11% 4 9% 5% 4 4 3 5% 6% 30%	114279
Transparency	Day	53% 5% 11% 19% 10% 1	118050
	Night	53% 5% 11% 20% 10% 1	118811
Seeing	Day	30% 17% 32% 16% 5% 1	17848
	Night	27% 15% 32% 20% 6%	25464

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	14% 14% 6% 16% 9% 6% 4 3 4 4 20%	3616
	11th to 20th	Day	14% 14% 6% 17% 8% 7% 6% 4 5% 5% 14%	3852
	21st to end	Day	15% 12% 5% 10% 5% 6% 5% 4 5% 5% 27%	4466
February	1st to 10th	Day	20% 14% 5% 10% 7% 5% 4 4 4 5% 22%	1800
	11th to 20th	Day	18% 12% 4 12% 8% 5% 5% 4 5% 4 24%	1753
	21st to end	Day	13% 14% 4 8% 6% 4 4 3 4 7% 32%	1524
March	1st to 10th	Day	24% 13% 4 11% 7% 4 4 3 3 4 23%	4930
	11th to 20th	Day	17% 16% 4 9% 4 4 3 3 4 6% 30%	4980
	21st to end	Day	26% 15% 5% 10% 5% 4 4 3 4 4 21%	5466
April	1st to 10th	Day	25% 15% 4 8% 4 4 3 3 4 5% 26%	2364
	11th to 20th	Day	21% 12% 2 6% 4 3 4 3 6% 6% 33%	2467
	21st to end	Day	23% 11% 3 6% 4 3 4 3 4 6% 34%	2535
May	1st to 10th	Day	25% 13% 3 9% 5% 4 4 3 5% 6% 23%	4124
	11th to 20th	Day	24% 11% 3 7% 4 4 4 3 4 6% 30%	4398
	21st to end	Day	19% 9% 3 6% 4 4 4 4 6% 8% 33%	4836
June	1st to 10th	Day	26% 10% 3 7% 4 4 3 4 5% 7% 26%	2660
	11th to 20th	Day	22% 8% 3 7% 4 4 4 3 4 6% 34%	2766
	21st to end	Day	25% 10% 3 7% 6% 5% 4 3 5% 7% 26%	2850
July	1st to 10th	Day	21% 9% 2 6% 5% 4 4 4 6% 8% 30%	4664
	11th to 20th	Day	17% 10% 3 8% 4 4 4 4 6% 8% 32%	4440
	21st to end	Day	20% 9% 3 6% 4 4 4 4 5% 8% 33%	4564
August	1st to 10th	Day	22% 10% 3 8% 4 4 4 4 6% 8% 27%	4105
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	4045
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 8% 46%	4378
September	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 5% 7% 43%	2280
	11th to 20th	Day	19% 6% 2 6% 4 4 4 3 4 6% 43%	2171
	21st to end	Day	18% 8% 3 7% 3 3 4 3 4 6% 41%	2052
October	1st to 10th	Day	19% 7% 2 6% 4 4 4 4 6% 8% 37%	1900
	11th to 20th	Day	20% 8% 2 6% 4 4 4 4 6% 8% 34%	1900
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 4 6% 33%	2090
November	1st to 10th	Day	20% 11% 3 7% 5% 4 3 3 5% 7% 32%	1654
	11th to 20th	Day	22% 13% 4 7% 4 4 4 4 5% 7% 26%	1600
	21st to end	Day	21% 11% 5% 9% 6% 5% 5% 4 5% 5% 26%	1600
December	1st to 10th	Day	24% 10% 5% 11% 5% 4 4 4 5% 6% 22%	2610
	11th to 20th	Day	28% 11% 3 7% 5% 4 4 3 4 6% 25%	2996
	21st to end	Day	21% 13% 6% 8% 5% 4 4 3 5% 6% 26%	3982
January	1st to 10th	Night	13% 14% 5% 17% 7% 5% 6% 4 5% 5% 18%	6349
	11th to 20th	Night	16% 15% 6% 16% 7% 5% 5% 3 5% 5% 17%	6026
	21st to end	Night	13% 12% 4 11% 7% 6% 5% 5% 6% 7% 24%	6490
February	1st to 10th	Night	20% 12% 5% 10% 6% 6% 5% 4 6% 5% 22%	2600
	11th to 20th	Night	17% 10% 5% 10% 7% 5% 5% 4 6% 6% 23%	2533
	21st to end	Night	13% 12% 4 9% 6% 5% 5% 4 6% 6% 30%	2100
March	1st to 10th	Night	25% 14% 4 8% 6% 5% 4 3 4 5% 24%	4910
	11th to 20th	Night	19% 12% 5% 8% 5% 5% 5% 3 5% 6% 27%	4979
	21st to end	Night	30% 12% 4 9% 4 3 3 2 3 5% 26%	5466
April	1st to 10th	Night	25% 13% 3 6% 4 4 4 4 4 5% 28%	2035
	11th to 20th	Night	24% 9% 3 6% 4 4 4 4 4 7% 32%	1879
	21st to end	Night	24% 10% 2 7% 4 4 4 3 4 6% 32%	1762
May	1st to 10th	Night	20% 16% 4 7% 4 4 4 3 5% 6% 28%	2817
	11th to 20th	Night	26% 11% 3 6% 4 4 4 3 5% 6% 28%	2506
	21st to end	Night	20% 11% 4 7% 5% 4 3 3 5% 6% 31%	2749
June	1st to 10th	Night	26% 9% 2 6% 4 4 4 3 5% 7% 30%	1520
	11th to 20th	Night	20% 8% 3 7% 5% 4 3 3 5% 6% 36%	1425
	21st to end	Night	27% 9% 3 7% 4 5% 3 3 6% 5% 27%	1330
July	1st to 10th	Night	20% 10% 3 7% 5% 5% 5% 4 6% 8% 28%	2269
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 6% 8% 32%	2530
	21st to end	Night	18% 7% 3 6% 4 4 4 4 6% 7% 38%	2989
August	1st to 10th	Night	21% 10% 3 7% 4 4 4 2 5% 6% 35%	2825
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	2782
	21st to end	Night	10% 5% 2 4 4 3 4 3 5% 7% 52%	3276
September	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 5% 7% 46%	1900
	11th to 20th	Night	18% 7% 2 4 3 3 4 3 5% 6% 46%	2007
	21st to end	Night	20% 9% 2 7% 3 4 3 3 4 6% 39%	2128
October	1st to 10th	Night	18% 6% 3 6% 5% 3 3 3 5% 6% 42%	2280
	11th to 20th	Night	20% 6% 2 5% 4 4 4 4 5% 7% 40%	2280
	21st to end	Night	23% 9% 2 5% 4 3 3 4 4 6% 36%	2508
November	1st to 10th	Night	19% 9% 2 6% 4 4 5% 3 6% 7% 35%	2640
	11th to 20th	Night	21% 11% 3 8% 5% 4 5% 3 5% 6% 28%	2800
	21st to end	Night	22% 10% 3 8% 6% 4 4 4 5% 7% 27%	2800
December	1st to 10th	Night	16% 11% 4 12% 6% 5% 5% 4 6% 8% 25%	4580
	11th to 20th	Night	25% 11% 2 6% 4 4 3 3 5% 6% 29%	5213
	21st to end	Night	24% 10% 4 8% 5% 4 4 3 4 5% 29%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	65% 5% 13% 12% 5%	3617
	11th to 20th	Day	67% 5% 12% 12% 3	3884
	21st to end	Day	54% 5% 10% 15% 14% 3	4466
February	1st to 10th	Day	63% 3 11% 15% 7% 1	1800
	11th to 20th	Day	62% 4 10% 16% 7% 1	1761
	21st to end	Day	53% 3 12% 20% 10% 3	1524
March	1st to 10th	Day	65% 3 8% 13% 8% 3	4934
	11th to 20th	Day	54% 4 14% 20% 7% 1	4980
	21st to end	Day	66% 4 5% 14% 10% 1	5466
April	1st to 10th	Day	61% 3 7% 14% 12% 2	2364
	11th to 20th	Day	51% 5% 8% 22% 13% 1	2467
	21st to end	Day	53% 3 10% 19% 13% 3	2535
May	1st to 10th	Day	60% 5% 12% 16% 7% 1	4138
	11th to 20th	Day	56% 6% 11% 19% 8% 1	4402
	21st to end	Day	46% 7% 15% 20% 11% 1	4836
June	1st to 10th	Day	57% 5% 10% 19% 9%	2660
	11th to 20th	Day	51% 4 11% 20% 13% 2	2766
	21st to end	Day	58% 6% 11% 15% 9% 1	2850
July	1st to 10th	Day	51% 5% 13% 17% 12% 2	4592
	11th to 20th	Day	49% 5% 13% 22% 9% 1	4364
	21st to end	Day	48% 6% 15% 20% 10% 1	4418
August	1st to 10th	Day	55% 6% 13% 18% 9% 1	4092
	11th to 20th	Day	46% 6% 14% 25% 9%	4084
	21st to end	Day	33% 5% 14% 27% 20% 1	4378
September	1st to 10th	Day	40% 4 13% 25% 16% 2	2280
	11th to 20th	Day	43% 4 8% 25% 18% 2	2171
	21st to end	Day	44% 5% 7% 26% 17% 1	2052
October	1st to 10th	Day	45% 6% 11% 25% 13% 1	3160
	11th to 20th	Day	38% 6% 10% 24% 21% 1	3140
	21st to end	Day	45% 4 13% 23% 12% 3	3428
November	1st to 10th	Day	53% 5% 11% 21% 10% 1	1654
	11th to 20th	Day	56% 4 13% 18% 8% 1	1600
	21st to end	Day	60% 4 12% 18% 6%	1600
December	1st to 10th	Day	59% 6% 12% 20% 3	2610
	11th to 20th	Day	60% 4 10% 16% 11%	2995
	21st to end	Day	57% 3 15% 16% 9%	3982
January	1st to 10th	Night	62% 7% 10% 16% 4	6349
	11th to 20th	Night	65% 6% 12% 13% 4	6068
	21st to end	Night	55% 6% 13% 18% 9%	6490
February	1st to 10th	Night	60% 5% 11% 15% 8% 1	2600
	11th to 20th	Night	58% 4 13% 17% 7% 1	2536
	21st to end	Night	53% 4 12% 21% 9% 2	2100
March	1st to 10th	Night	64% 3 6% 13% 12% 1	4919
	11th to 20th	Night	57% 4 13% 18% 7% 1	4980
	21st to end	Night	62% 3 7% 17% 10% 1	5466
April	1st to 10th	Night	57% 4 8% 15% 12% 3	2035
	11th to 20th	Night	53% 4 7% 22% 13% 2	1879
	21st to end	Night	54% 3 10% 19% 12% 3	1762
May	1st to 10th	Night	58% 4 13% 17% 7%	2823
	11th to 20th	Night	57% 5% 9% 20% 7% 1	2507
	21st to end	Night	52% 6% 12% 21% 8% 1	2750
June	1st to 10th	Night	55% 4 11% 21% 9%	1520
	11th to 20th	Night	50% 5% 11% 22% 12% 1	1425
	21st to end	Night	58% 6% 11% 15% 10% 1	1330
July	1st to 10th	Night	53% 6% 12% 18% 10% 1	2233
	11th to 20th	Night	50% 6% 14% 19% 10% 1	2482
	21st to end	Night	46% 6% 12% 22% 13% 1	2895
August	1st to 10th	Night	51% 5% 13% 22% 9%	2813
	11th to 20th	Night	46% 6% 16% 24% 8%	2809
	21st to end	Night	31% 6% 14% 28% 21%	3275
September	1st to 10th	Night	38% 6% 10% 27% 17% 2	1900
	11th to 20th	Night	39% 4 8% 29% 17% 2	2007
	21st to end	Night	46% 5% 7% 25% 15% 1	2128
October	1st to 10th	Night	44% 5% 12% 24% 13% 1	3800
	11th to 20th	Night	34% 4 11% 28% 22%	3779
	21st to end	Night	40% 3 12% 25% 19% 1	4120
November	1st to 10th	Night	48% 5% 14% 21% 11% 1	2640
	11th to 20th	Night	54% 5% 11% 20% 9% 1	2800
	21st to end	Night	55% 5% 16% 18% 7%	2800
December	1st to 10th	Night	52% 7% 13% 24% 4	4580
	11th to 20th	Night	54% 4 12% 19% 11% 1	5215
	21st to end	Night	57% 4 12% 18% 9% 1	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 19% 32% 11% 3	400
	11th to 20th	Day	36% 18% 29% 14% 4	400
	21st to end	Day	27% 23% 34% 13% 3	440
February	1st to 10th	Day	34% 21% 30% 13% 2	400
	11th to 20th	Day	38% 23% 27% 10% 2	396
	21st to end	Day	32% 20% 30% 14% 3 1	347
March	1st to 10th	Day	36% 23% 30% 8% 2	500
	11th to 20th	Day	33% 24% 28% 10% 3 1	500
	21st to end	Day	38% 21% 27% 11% 4	550
April	1st to 10th	Day	42% 23% 26% 7% 2	500
	11th to 20th	Day	34% 23% 30% 10% 2	492
	21st to end	Day	33% 18% 32% 13% 4	480
May	1st to 10th	Day	37% 22% 29% 10% 2 1	468
	11th to 20th	Day	29% 19% 29% 18% 4 1	674
	21st to end	Day	31% 14% 35% 16% 3 1	735
June	1st to 10th	Day	32% 15% 31% 17% 4	680
	11th to 20th	Day	26% 15% 34% 20% 5% 1	680
	21st to end	Day	33% 12% 30% 19% 5% 1	680
July	1st to 10th	Day	24% 9% 36% 24% 7%	670
	11th to 20th	Day	21% 7% 39% 22% 9% 1	668
	21st to end	Day	24% 9% 38% 22% 7% 1	590
August	1st to 10th	Day	18% 10% 39% 24% 8% 1	486
	11th to 20th	Day	20% 9% 38% 21% 11% 2	479
	21st to end	Day	19% 11% 35% 25% 7% 3	539
September	1st to 10th	Day	20% 13% 37% 21% 7% 1	490
	11th to 20th	Day	26% 14% 29% 20% 8% 3	490
	21st to end	Day	23% 14% 35% 18% 7% 2	470
October	1st to 10th	Day	23% 13% 34% 20% 8% 3	390
	11th to 20th	Day	28% 16% 33% 15% 5% 4	390
	21st to end	Day	31% 17% 32% 13% 6% 1	429
November	1st to 10th	Day	26% 19% 31% 15% 7% 2	395
	11th to 20th	Day	41% 18% 24% 12% 5% 1	400
	21st to end	Day	35% 19% 32% 10% 4 1	400
December	1st to 10th	Day	39% 17% 29% 12% 3	400
	11th to 20th	Day	44% 21% 22% 13% 1	400
	21st to end	Day	36% 23% 26% 13% 2	440
January	1st to 10th	Night	32% 20% 31% 14% 3	900
	11th to 20th	Night	31% 18% 30% 18% 3	837
	21st to end	Night	26% 23% 32% 15% 4	880
February	1st to 10th	Night	32% 18% 31% 17% 2	800
	11th to 20th	Night	31% 23% 32% 13% 1	783
	21st to end	Night	22% 21% 35% 18% 4	660
March	1st to 10th	Night	29% 21% 33% 15% 2	800
	11th to 20th	Night	26% 20% 30% 19% 5% 1	800
	21st to end	Night	29% 21% 31% 14% 4	880
April	1st to 10th	Night	38% 15% 31% 13% 3	635
	11th to 20th	Night	29% 20% 32% 16% 3	594
	21st to end	Night	30% 18% 29% 19% 4 1	588
May	1st to 10th	Night	31% 15% 33% 16% 5% 1	570
	11th to 20th	Night	29% 12% 32% 20% 6% 1	562
	21st to end	Night	23% 14% 40% 18% 5% 1	616
June	1st to 10th	Night	32% 10% 34% 20% 4	570
	11th to 20th	Night	23% 12% 38% 21% 6%	475
	21st to end	Night	31% 7% 31% 23% 7% 1	380
July	1st to 10th	Night	20% 7% 38% 25% 10%	402
	11th to 20th	Night	19% 7% 38% 26% 10%	552
	21st to end	Night	21% 6% 30% 33% 9%	623
August	1st to 10th	Night	18% 7% 32% 32% 11% 1	568
	11th to 20th	Night	21% 7% 31% 28% 12% 1	562
	21st to end	Night	16% 7% 31% 32% 12% 1	627
September	1st to 10th	Night	19% 7% 35% 28% 10%	570
	11th to 20th	Night	22% 9% 30% 23% 13% 2	677
	21st to end	Night	26% 10% 32% 22% 9% 2	760
October	1st to 10th	Night	23% 9% 32% 26% 9% 1	760
	11th to 20th	Night	23% 15% 32% 20% 9% 1	760
	21st to end	Night	29% 15% 31% 19% 5%	836
November	1st to 10th	Night	28% 14% 30% 21% 6% 1	847
	11th to 20th	Night	32% 18% 27% 19% 5%	900
	21st to end	Night	30% 21% 27% 16% 5%	900
December	1st to 10th	Night	32% 16% 32% 14% 5%	900
	11th to 20th	Night	35% 21% 27% 14% 3	900
	21st to end	Night	32% 19% 28% 18% 3	990

Page generated at 2025-04-23 10:25:04. Version ac07

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